Lamar Technologies BETA D-50 - Manual (Page 13)

4. DETAILED OPERATING INSTRUCTIONS
The discharge of a lead-acid battery beyond the point of which exhaustion of the cell is
approached can be harmful, as the battery may form a sulfate deposit on its plates. This is
particularly true if the battery is not soon recharged.
4.2 DISCHARGE CAPACITY TESTING
The purpose of a discharge test is to determine the long-term capacity of the battery. For
a lead-acid aircraft battery, for example, the capacity test is the amount of current which
can be delivered for one hour or until the voltage decreases to 1.5 volts per cell. This is
considered the emergency capacity of the battery. The result of the test will
determine whether to accept or reject the battery and which steps could be taken to
electrically recondition the battery.
The amount of electrical capacity available from a fully charged nickel-cadmium or leadacid
battery is defined by the capacity rating of the battery and is stated in terms of
ampere hours. Because of internal resistance, the higher the discharge rate (higher
current during a shorter period of time) demanded of a battery, the less usable capacity it
can supply. See figure 4-2.
4.2.1 Constant-Current Discharge Method
The most accurate and repeatable method of measuring capacity is to discharge the
battery at a constant-current rate. This is also the method used by the Beta D-50. The
load resistance in this method is continuously and automatically varied to maintain a
constant discharge current as the battery's voltage decreases. A schematic of the
circuit diagram for the Beta D-50 can be seen in figure 4-3. At the end of discharge, the
calculation of ampere-hour capacity is the product of discharge current and the elapsed
discharge time.
A
AC
line
SOLID
BATTERY V STATE
CONTROL
CIRCUITS
LOAD RESISTOR


Figure 4-3. Circuit diagram for constant current discharge test
NOTE: Always read the battery manufacturer's operating
instructions. Some lead-acid batteries are recommended at
a 20 volt cutoff when tested for an hour at 80% of rated
capacity. Some nickel-cadmium battery requirements are
testing for two hours at half the rated capacity, and a 1 volt
per cell cutoff.
BETA D-50 TESTER / ANALYZER - OPERATING MANUAL V2.2 Page 11